The present invention relates to a synchronizing mechanism for clutches, in particular for manual gearboxes for motor vehicles, comprising a guide sleeve fixed to a shaft and provided with an external toothing, a gear which is rotatably seated on the shaft beside the said guide sleeve and provided with a clutch body having coupling teeth forming an external toothing, the said clutch body being fastened to the said gear to rotate therewith, further a gearshift sleeve surrounding the guide sleeve and provided with an inner toothing which is in engagement with the outer toothing of the guide sleeve and which comprises coupling teeth that can be brought into engagement with the external toothing of the coupling body by axial displacement of the gearshift sleeve, a synchronizer ring provided between the guide sleeve and the coupling body, the said synchronizer ring engaging, in the circumferential direction, the gearshift sleeve in form-locking relationship and the clutch body in frictional relationship and carrying on its circumference locking teeth coacting with locking teeth that belong to the internal toothing of the gearshift sleeve and exhibit an axial length smaller than the length of the coupling teeth, and wedge surfaces provided at the ends of all teeth facing each other, the wedge surfaces of the locking teeth being in contact with each other when no synchronism exists so as to prevent in this manner the coupling teeth from engaging each other as long as synchronism has not been reached.
A synchronizing mechanism of this type has been known before from German Patent Specification No. 26 59 448. The use of different teeth for locking the gearshift sleeve as long as synchronism has not been reached and for establishing the form-locking engagement between the gearshift sleeve and the gear to be engaged (change gear), enables very short shifting travels to be reached without impairing the safety of the shifting operation. When during the shifting operation the speed of the shaft and of the change gear have been adapted to each other, as a result of the synchronizer ring being urged against the clutch body and the moment of friction exerted thereby, and when the moment of friction breaks down suddenly when synchronism is reached, then the gearshift sleeve can rotate the synchronizer ring and the change gear relative to the gearshift sleeve, due to the wedge surfaces provided at the ends of its locking teeth and coacting with the wedge surfaces at the ends of the locking teeth of the synchronizer ring, until the locking teeth of the gearshift sleeve face the gaps between the locking teeth of the synchronizer ring so that the gearshift sleeve can be displaced across the synchronizer ring until the coupling teeth of the gearshift sleeve come to engage the coupling teeth of the clutch body. Considering that the angular position of the synchronizer ring relative to the clutch body is undefined, the coupling teeth of the gearshift sleeve will normally not face the gaps between the coupling teeth of the clutch body so that the coupling teeth will come to bear against each other by their--more or less overlapping--wedge surfaces and any further axial displacement of the gearshift sleeve will be possible only if a rotary movement occurs between the synchronizer ring and the clutch body, i.e. when the frictional engagement between the synchronizer ring and the change gear is released. This necessity to unlock the two elements leads to a certain resistance during engagement of the gear, which makes itself felt unfavorably in the form of a second pressure point at the end of the shifting travel.